The present invention relates to an accelerator apparatus, being configured for accelerating charged particles with pulsed radiation, preferably in a THz or RF wavelength range, in particular including a horn-shaped coupling device and a waveguide device with a particle injection section. Furthermore, the present invention relates to a charged particle gun including the accelerator apparatus, and to a method of accelerating charged particles, like e.g., electrons or protons. Applications of the invention are available in the field of compact particle accelerators, e.g., included in X-ray sources, or for electron diffraction imaging.
For describing the background of the invention, particular reference is made to the following publications:    [1] E. A. Nanni et al. in “Nature Communications” 6, 696 (2015);    [2] L. J. Wong et al. in “Optics Express” 21, 9792-9806 (2013);    [3] R. Yoder et al. in “Physical Review Special Topics-Accelerators and Beams” 8, 111301 (20051); and    [4] E. A. Nanni et al. in “Proceedings of IPAC2014”, Dresden, Germany, WEOAB03, ISBN 978-3-95450-132-8, p. 1896;    [5] European patent application No. 15001303.5 (not published on the priority date of the present specification); and    [6] S.-W. Huang, et al., Opt. Lett. 38 (2013) 796-798.
Various concepts of electron acceleration using electric fields are generally known, e.g., based on a static potential (Cockcroft-Walton accelerator, Van de Graaff accelerator) or based on microwave or millimeter-wave oscillating fields (circular particle accelerators, like cyclotron, betatron, and synchrotron; or linear particle accelerators, like linac). Although these systems represent highly developed techniques with strong acceleration capabilities, they have substantial shortcomings in terms of efficiency, size and complexity of operation, resulting in limited applications, e.g., in research on basic and applied sciences. However, there is an interest in more efficient and compact devices.
Recent developments in ultrafast techniques have influenced the development of compact electron accelerators, based on e.g., dielectric laser acceleration (DLA), laser-driven plasma acceleration (LPA), and laser wake-field acceleration (LWFA). These accelerators employ strong optical fields created with high power lasers for the particle acceleration. The high operation frequency of infra-red (IR) lasers as well as the available high electric fields, e.g. based on chirped pulse amplification makes this range of the electromagnetic spectrum promising for realization of small size accelerators. However, the acceleration schemes based on optical pulses suffer from difficulties caused by the short optical wavelengths.
Another promising progress in the ultrafast techniques is the development of single cycle THz pulse sources using optical rectification. The resulting high power THz pulses can also be used to develop compact accelerators ([1] to [5]). When compared to direct optical acceleration, the benefit is then a longer wavelength and relaxed limitations in the amount of charge per bunch. According to [4] and [5], radially polarized THz pulses are coupled into a waveguide using a centrally loaded dielectric horn. However, these techniques have limitations as the waveguide used is a single mode waveguide being resonant for a central frequency of the THz pulses only. Therefore, the proposed structures are not suitable in the regime of ultrashort pulses. More accurately, the travelling wave cavities and waveguides should be fed with at least 10 to 50 cycle pulses, due to their inherent resonant behavior.
Electron acceleration is required in particular in the electron gun of a compact X-ray source. In consideration of the need for compact X-ray sources replacing available hard X-ray sources, like Free Electron Lasers (FEL), e.g., in X-ray spectroscopy and imaging, there is a particular need for compact electron accelerators and guns. In the past decade, the attempts to devise a small electron gun were mainly centered on increasing the operation frequency of the device. This leads to smaller structures functioning in shorter wavelengths. The damage threshold of the structure was the main obstacle preventing the miniaturization of electron guns. It was observed that this limitation is strongly relaxed when shorter pulses, i.e. broadband excitations are used. However, the conventional electron guns could not operate based on ultrashort pulses. This difficulty calls the need to develop new schemes for electron acceleration based on short pulses.